1. Field of the Invention
The present invention relates to a composite optical disk structure in which reproduction is carried out from the one side or both sides of a composite optical disk structure consisting of two single-side disk plates bonded to each other.
2. Description of the Prior Art
There has been proposed an optical disk structure composed of two sheets of bonded disk plates for reading recorded information by using laser light. Such an optical disk structure includes a laser disk structure and DVD (digital video disk). An explanation will be given of a process of fabricating a conventional laser disk structure.
First, with a metallic mold (referred to as "stamper"), on which pits bearing recorded information are formed previously, attached to a resin molding machine such as an injection molding machine, a transparent resin material of polycarbonate or acrylic is injection-molded or pressure-molded to form a transparent disk plate (single-side disk plate) which is a transparent disk with pits duplicated (plate molding step). The actual plastic disk (single-side disk plate) fabricated by the plate molding step contains a ring-shaped concave groove (hereinafter referred to as "stamper clamping groove) which abuts on the stamper clamp for clamping the stamper to the resin molding machine.
A metallic material such as aluminum is applied to the pit face of the disk plate by sputtering or vacuum evaporation to form a reflection film (reflection film making step). A protection film of hot-melt type is applied to the surface of the reflection film by a roll coating machine to form a protection film (5-30 .mu.m) for preventing damage or corrosion (protection film applying step).
Thus, a pair of single-side disk plates having surfaces A and B to be bonded to each other are individually formed. Thereafter, thermoplastic adhesive called "hot-melt" having a thickness of 5-30 .mu.m) is applied to the bonding surfaces A and B of the respective single-side disk plates A and B (adhesive applying step).
A pair of single-side disk plates with adhesive applied are bonded to each other in such a manner as shown in FIG. 8 (bonding step). Specifically, the adhesive-applied bonding surfaces of a pair of single-side disk plates 10a and 10b which are mated with each other are placed on a lower-side crimping table 51 having a flat pressing face. Thereafter, an upper crimping table 52 is lowered by a pressure cylinder 53 so that the pair of single-side disk plates 10a and 10b are pressure-crimped by the flat pressing face into one sheet of laser disk.
In such a laser disk structure, adhesive of hot-melt type is applied to the other portion than a center hole, and solidified by cooling. Thereafter, the disk surfaces are bonded to each other by means of the solidified adhesive. Therefore, there is weak possibility that the adhesive sticks or leaks out toward the center hole.
In recent years, a digital video disk structure called "DVD" of a double-side recording system having a small size (diameter: 12 cm and thickness of the one side of the optical disk: 0.6 mm) permitting high density recording has been developed. The DVD can be fabricated by the same manner as in the process as described above except that ultraviolet rays setting resin is applied onto the surface of the protection film by using a spinner.
It is well known that the recording capacity of the DVD can be increased by a "double layer recording". The double layer recording will be carried out as follows. As shown in FIG. 6A, a total internal reflection film 114 made of e.g. aluminum is formed on the one recording surface of a single-side disk plate 101 and a semi-transparent film 124 of e.g. dielectric is formed on the one recording surface of another single-side optical disk plate 102. These recording surfaces are bonded apart from each other by a prescribed interval through an ultraviolet rays setting resin 16. The ultraviolet rays setting resin 16 serves as a bonding layer and a protection layer.
Information reading from the recording single-side reproduction type composite disk structure thus formed is carried out as follows. As shown in FIG. 7, the laser light 133 emitted from a semiconductor laser 131 is converged on the semi-transparent film 124 serving as a first recording surface by a converging lens 132 to read the information by means of the light reflected therefrom. The information on the total internal reflection film serving as a second recording surface can be read in such a manner that the converging lens 132 is moved to a prescribed location 132 toward the composite optical disk structure, laser light 135 is converged on the second recording surface 114 as spot light to read the information by means of the light reflected therefrom.
FIG. 5A is a plan view of first and second single-side disk plates; and FIG. 5B is a sectional view thereof. As shown in FIGS. 5A and 5B, the first single-side optical disk plate 101 constituting a part of the double-layer recording type composite optical disk structure includes a disk clamp area C around a center hole 110, a stamper clamp groove 113 around the disk clamp area C, an data recording area 115 which is an information recording area around the stamper clamp groove 113, a total internal reflection film 115 covering the data recording area 114 and a signal-free area 117 around the data recording area 114.
Likewise, the second single-side optical disk plate 102 constituting a part of the recording type composite optical disk structure also includes a disk clamp area C around a center hole 120, a stamper clamp groove 123 around the disk clamp area C, an data recording area 125 which is an information recording area around the stamper clamp groove 123, a semi-transparent film 125 covering the data recording area 124 and a signal-free area 127 around the data recording area 124.
In the double-layer recording type composite optical disk structure, the opaque hot-melt which was used to bond the laser disk plates as described previously cannot be used to bond the first and second single-side optical disk plates 101 and 102 to each other. In place of it, an ultraviolet rays setting type adhesive which can transmit light can be used. This is because ultraviolet rays can be irradiated through the semitransparent film 124 constituting the first recording surface.
Thus, as shown in FIG. 6A, such an ultraviolet-rays setting type adhesive 16 is filled between the total internal reflection film 114 and the semitransparent film 124 by a spin-coating method to bond the pair of optical disk plates 101 and 102. In such a double-layer recording type composite optical disk structure, in order that either one of two data recording areas 115 and 125 can be selectively reproduced, i.e., two recording areas are not simultaneously located within the focusing depth of the convergence lens 132, the ultraviolet rays setting type resin layer 16 must have a thickness of 30-40 .mu.m.
However, in the conventional double-layer recording type composite optical disk structure, it was difficult to bond the total internal reflection film 114 and the semi-transparent film 124 apart from each other by a prescribed interval (e.g. 40 .mu.m).
Further, as shown in FIG. 6B, in bonding the one-side optical disk plates 101 and 102 to each other, the adhesive 16 therebetween may stick out into the center holes 110 and 120 so that the stick-out portion 16a is solidified, thus deforming the internal shapes of the center holes 110 and 120.
Now referring to FIGS. 9A to 9E, this problem will be explained in more detail.
The double-layer recording type composite disk structure has been fabricated as explained below. First, a first single-side disk 221 is prepared in which a center hole 221a, a clamping area 221c, a clamping area back face 221c', a stamper clamp groove 221d and pits for information recording are formed on a transparent plastic disk plate 221p. A total internal reflection film 221e is vacuum evaporated on the recording area where the pits for information recording are formed.
Likewise, a second single-side disk plate 222 is prepared in which a center hole 222a, a clamping area 222c, a clamping area back face 222c', a stamper clamp groove 222d and pits for information recording are formed on a transparent plastic disk plate 222p. A semi-transparent film 222e is vacuum evaporated on the recording area where the pits for information recording are formed.
In FIG. 9B, an ultraviolet-rays setting type adhesive 203 is dropped and applied, near the center hole 221a, on the bonding face of the first single-side disk plate 221. In FIG. 9C, the second single-side disk 222 with its bonding face downward is placed on the first single-side disk plate 221.
In FIG. 9D, the upper (first) and lower (second) single-side disk plates 221 and 222 are rotated so that they are intimately brought into contact with each other while enlarging redundant adhesive 203 outwardly in the radial direction of the disk plates.
In FIG. 9E, ultraviolet rays are radiated by an ultraviolet rays lamp 204 to harden the ultraviolet-rays setting type adhesive 203. Thus, the composite optical disk structure which permits data to be reproduced from one side thereof can be fabricated.
The composite optical disk structure thus fabricated has the following defect. Specifically, as seen from FIG. 9D, because of load of the single-side disk plate 222 when it is placed on the single-side disk plate 221, the ultraviolet-rays setting type resin 203 applied at the outer periphery of the center holes 221a, 222a may leak out into the center holes 221a and 222a, thus deforming the shapes of these holes. As a result, several problems, e.g. "shaft divergence" that when the disk structure is set in a player, a rotary shaft fluctuates, and "face fluctuation" that the rotary shaft and the center axis of the disk are inclined, occur.